Production method for sintered machine components

ABSTRACT

A production method for sintered machine components, includes preparing an Fe alloy powder A, an Fe alloy powder B, an Fe—P powder, and a graphite powder. The Fe alloy powder A consists of, by mass %, 25 to 45% of Cr, 1.0 to 3.0% of Mo, 1.0 to 3.0% of Si, 0.5 to 1.5% of C, and the balance of Fe and inevitable impurities. The Fe alloy powder B consists of, by mass 15 to 35% of Cr, 15 to 30% of Ni, and the balance of Fe and inevitable impurities, and the Fe—P powder consists of 10 to 30 mass % of P and the balance of Fe and inevitable impurities. The production method further includes mixing 40 to 60 mass % of the Fe alloy powder B, 1.0 to 5.0 mass % of the Fe—P powder, and 0.5 to 3.5 mass % of the graphite powder with the Fe alloy powder A into a mixed powder. The production method further includes compacting the mixed powder into a green compact and sintering the green compact.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a production method for sintered machine components; the production method may be preferably used for, for example, turbo components of turbochargers, and specifically, nozzle bodies that must have heat resistance, corrosion resistance, and wear resistance.

2. Background Art

In general, in a turbocharger fixed to an internal combustion engine, a turbine is rotatably supported by a turbine housing connected to an exhaust manifold of the internal combustion engine, and plural nozzle vanes are rotatably supported such that the nozzle vanes surround the outer circumference of the turbine. Exhaust gas flowing in the turbine housing flows from the outer circumference of the turbine into the turbine and is discharged in the axial direction, thereby rotating the turbine. A compressor is provided at the same shaft as the shaft of the turbine and is at a side opposite to the side with the nozzle vanes. Then, the compressor is rotated, whereby air to be supplied to the internal combustion engine is compressed.

The nozzle vane is rotatably supported by a ring-shaped part called a “nozzle body” or a “nozzle mount”. The shaft of the nozzle vane penetrates the nozzle body and is connected to a link structure. By driving the link structure, the nozzle vane is turned, and a degree to which a flow path is open is adjusted to allow exhaust gas to flow into the turbine. The present invention relates to turbo components that may be provided at a turbine housing, such as a nozzle body (nozzle mount) and a nozzle plate to be mounted on the nozzle body.

Since the above-described turbo components for turbochargers may be subjected to corrosive exhaust gas at high temperatures, the turbocharger must have heat resistance and corrosion resistance. In addition, since the turbocharger slidingly contacts a nozzle vane, the turbocharger must also have wear resistance. Therefore, a high Cr steel, a wear resistant material, and the like are conventionally used. The wear resistant material may be formed by performing a chromium surface treatment on a SCH22-type material, as specified by the JIS (Japanese Industrial Standards), in order to improve corrosion resistance. As a wear resistant component that has superior heat resistance, corrosion resistance, and wear resistance, and that is inexpensive, a wear resistant component including carbides dispersed in a matrix of a ferrite stainless steel has been suggested (for example, see Japanese Patent No. 3784003).

SUMMARY OF THE INVENTION

In recent years, there is a trend toward increasing the speed of internal combustion engines and increasing the output therefrom. Therefore, for turbo components of turbochargers, a wear resistant member having further improved heat resistance, corrosion resistance, wear resistance, and high-temperature strength, is required. The components of turbochargers are typically made of an austenitic heat resistant material. On the other hand, a turbo component for a turbocharger disclosed in Japanese Patent No. 3784003 is made of a ferritic material. In this case, the turbo component has a different thermal expansion coefficient from that of surrounding members, whereby the design of the turbo component is difficult for practical use. Therefore, the turbo component is required to have a similar thermal expansion coefficient as that of the surrounding austenitic heat resistant material. An object of the present invention is to provide a production method for wear resistant components that satisfy the above requirements.

The present invention provides a production method for sintered machine components, and the production method includes preparing an Fe alloy powder A, an Fe alloy powder B, an Fe—P powder, and a graphite powder. The Fe alloy powder A consists of, by mass %, 25 to 45% of Cr, 1.0 to 3.0% of Mo, 1.0 to 3.0% of Si, 0.5 to 1.5% of C, and the balance of Fe and inevitable impurities. The Fe alloy powder B consists of, by mass %, 15 to 35% of Cr, 15 to 30% of Ni, and the balance of Fe and inevitable impurities, and the Fe—P powder consists of 10 to 30 mass % of P and the balance of Fe and inevitable impurities. The production method further includes mixing 40 to 60 mass % of the Fe alloy powder B, 1.0 to 5.0 mass % of the Fe—P powder, and 0.5 to 3.5 mass % of the graphite powder with the Fe alloy powder A into a mixed powder. The production method further includes compacting the mixed powder into a green compact and sintering the green compact.

In the production method for sintered machine components of the present invention, the Fe alloy powder A includes a substantial amount of elements for improving wear resistance, and the Fe alloy powder B is soft and improves compressibility of the Fe alloy powder A. The alloying elements are divided into the Fe alloy powder A and the Fe alloy powder B and are added together, whereby compressibility of the raw powder is improved. In order to densify the sintered compact, the liquefying temperature of the mixed powder of the Fe alloy powder A and the Fe alloy powder B is reduced so that a liquid state is generated in sintering. Therefore, P and C are used in the form of an Fe—P powder and the graphite powder, respectively, and the Fe—P powder and the graphite powder are mixed with the Fe alloy powder A and the Fe alloy powder B, whereby a mixed powder is formed. Hereinafter, the reasons for limiting the above amounts and functions of the present invention are described. In the following descriptions, the symbol “%” represents “mass %”.

Cr:

Cr improves heat resistance and corrosion resistance of a matrix, and Cr also improves wear resistance when combined with C into carbides. In order to uniformly improve a matrix by such effects of Cr, Cr is added to the mixed powder in the form of an Fe alloy powder. If the amount of Cr in the Fe alloy powder A is less than 25%, and the amount of Cr in the Fe alloy powder B is less than 15%, precipitation amount of Cr carbides is small, whereby wear resistance will be insufficient, and heat resistance and corrosion resistance of a matrix are decreased. On the other hand, if the amount of Cr in the Fe alloy powder A is more than 45%, the compressibility of the raw powder is extremely decreased. Therefore, the upper limit of the amount of Cr in the Fe alloy powder A must be 45%. In order to form the Fe alloy powder B so that it is soft, the upper limit of the amount of Cr in the Fe alloy powder B must be 35%. Accordingly, the amount of Cr in the Fe alloy powder A is set to be 25 to 45%, and the amount of Cr in the Fe alloy powder B is set to be 15 to 35%. Since the Fe alloy powder B must be softer than the Fe alloy powder A, the amount of Cr in the Fe alloy powder B must be less than the amount of Cr in the Fe alloy powder A.

Mo:

Mo improves heat resistance and corrosion resistance of a matrix, and Mo also improves wear resistance when combined with C into carbides. If Mo is added to the mixed powder in the form of a pure metal powder (molybdenum powder), Mo is not easily uniformly dispersed into the entirety of a matrix because Mo disperses slowly during sintering. Therefore, Mo is preferably added to the mixed powder in the form of an Fe alloy powder. In view of this, in the production method for sintered machine components of the present invention, Mo is added to and is solid solved in the Fe alloy powder A. If the amount of Mo in the Fe alloy powder A is less than 1.0%, the effects of Mo for improving heat resistance and corrosion resistance of a matrix are insufficient. On the other hand, if the amount of Mo in the Fe alloy powder A is more than 3.0%, the effects of Mo are not effectively obtained. Accordingly, the amount of Mo in the Fe alloy powder A is set to be 1.0 to 3.0%.

Si:

The Fe alloy powder A includes a large amount of Cr that is easily oxidizable, compared to the Fe alloy powder B, and therefore, it is effective to add Si as a deoxidizing agent in producing the Fe alloy powder A. In addition, Si improves sinterability. Therefore, an appropriate amount of Si is added to and is solid solved in the Fe alloy powder A. If the amount of Si in the Fe alloy powder A is less than 1.0%, the effects of Si are insufficient. On the other hand, if the amount of Si in the Fe alloy powder A is more than 3.0%, the hardness of the Fe alloy powder A is greatly increased, whereby the compressibility of the raw powder is extremely decreased. Accordingly, the amount of Si in the Fe alloy powder A is set to be 1.0 to 3.0%. If Si is added to and is solid solved in the Fe alloy powder B, the hardness of the Fe alloy powder B is increased, whereby the effect for improving the compressibility of the Fe alloy powder A is decreased. Therefore, Si is not added and is not solid solved in the Fe alloy powder B. Since Si can be used as a deoxidizing agent in producing the powder, not more than 1.0% of Si may be included in the Fe alloy powder B as an impurity.

Ni:

Ni disperses in a matrix and thereby has an effect of solid-solution strengthening, and Ni austenitizes the matrix, whereby Ni improves high-temperature strength of wear resistant components. In order to uniformly improve the entirety of a matrix by the effects of Ni, Ni is preferably added to the mixed powder in the form of an Fe alloy powder. When Ni is added to and is solid solved in the Fe alloy powder, hardness of the Fe alloy powder is not greatly increased. In view of this, in the production method for sintered machine components of the present invention, Ni is added to and is solid solved in the Fe alloy powder B. If the amount of Ni in the Fe alloy powder B is less than 15%, the high-temperature strength of a sintered compact will be insufficient, and corrosion resistance of the sintered compact is decreased. On the other hand, even if the amount of Ni is more than 30%, the high-temperature strength of the sintered compact is not further improved. Accordingly, the amount of Ni in the Fe alloy powder B is set to be 15 to 30%.

P:

P and C generate an Fe—P-C liquid phase in sintering and thereby facilitate densification of a sintered compact. Therefore, a density ratio of 90% or higher can be achieved. In order to facilitate liquefaction in sintering so as to densify a sintering compact, P is added to the mixed powder in the form of an Fe—P alloy powder. If the amount of P in the Fe—P powder is less than 10%, a liquid phase is not sufficiently generated, and the density of a sintered compact is not improved. On the other hand, if the amount of P is more than 30%, the hardness of the Fe—P power is greatly increased, whereby the compressibility of the Fe—P powder is extremely decreased.

When the amount of the Fe—P powder in the mixed powder is less than 1.0%, a liquid phase is not sufficiently generated, whereby densification is not sufficiently performed, and a density ratio will be less than 90%. On the other hand, when the amount of the Fe—P alloy powder is more than 5.0%, too much of a liquid phase is generated, whereby a compact may be deformed in sintering. In this case, the maximum amount of P in the overall composition is 1.5%. As described above, 1.0 to 5.0% of an Fe—P alloy powder including 10 to 30% of P is added to a mixed powder.

C:

C can lower a liquefying temperature, thereby generating an Fe—P—C liquid phase in sintering and facilitating densification of a sintered compact. In addition, C improves wear resistance when combined with Cr or Mo into carbides. In a case of adding the entire amount of C in the form of a graphite powder, an Fe alloy powder includes Cr and Mo that are solid solved in the Fe matrix, and the Fe alloy powder is too hard, whereby the compressibility of the Fe alloy powder is decreased. Use of a large amount of the graphite powder also causes decrease in the compressibility of the mixed powder. Therefore, a partial amount of C is added to the mixed powder in the form of an Fe alloy powder, and the remaining amount of C is added to the mixed powder in the form of a graphite powder. In this case, since the Fe alloy powder B must be soft, a partial amount of C is added to and is solid solved in the Fe alloy powder A. When a partial amount of C is added to the mixed powder in the form of an Fe alloy powder A, Cr and Mo in the Fe alloy powder A precipitate in the Fe alloy powder A as carbides, whereby the amounts of Cr and Mo solid solved in the matrix of the Fe alloy powder A are decreased, and the compressibility of the Fe alloy powder A is improved. Moreover, by adding the remaining amount of C to the mixed powder in the form of a graphite powder, the compressibility of the mixed powder is improved. If the amount of C in the Fe alloy powder A is less than 0.5%, the amounts of Cr and Mo solid solved in the Fe alloy powder A are increased, whereby the hardness of the Fe alloy powder A is increased, and the compressibility of the Fe alloy powder A is decreased. On the other hand, when the amount of Cr is more than 1.5%, the amount of carbides precipitated in the Fe alloy powder A is too great, whereby the hardness of the Fe alloy powder A is increased. Therefore, the amount of C in the Fe alloy powder A is set to be 0.5 to 1.5%.

A certain amount of C, which is required for forming carbides of Cr and Mo, is added to and is solid solved in the Fe alloy powder A, and the remaining amount of C is added to the mixed powder in the form of a graphite powder. A part of the graphite powder is used for reducing oxide layers on the surfaces of the Fe alloy powder particles during sintering, and therefore, an extra amount of the graphite powder must be added to the mixed powder. Since approximately 0.2% of graphite may be used for reduction during sintering, the amount of the graphite powder is preferably set to be 0.5% or more. On the other hand, if the graphite powder is excessively added, the matrix will be brittle. Moreover, the precipitation amount of carbides is increased, whereby mated materials such as vanes may be worn, and the Cr amount in the matrix is decreased, thereby decreasing heat resistance and corrosion resistance. Accordingly, the upper limit to the amount of the graphite powder is set to be 3.5%.

As described above, the Fe alloy powder A has a composition consisting of 25 to 45% of Cr, 1.0 to 3.0% of Mo, 1.0 to 3.0% of Si, 0.5 to 1.5% of C, and the balance of Fe and inevitable impurities, and the Fe alloy powder B has a composition consisting of 15 to 35% of Cr, 15 to 30% of Ni, and the balance of Fe and inevitable impurities. If the amount of the Fe alloy powder B is less than 40% with respect to the Fe alloy powder A, the effect for improving the compressibility of the raw powder is insufficient. In addition, the amount of Ni in a sintered machine component will be insufficient, whereby high-temperature strength is insufficient. On the other hand, if the amount of the Fe alloy powder B is more than 60%, the amount of the Fe alloy powder A is insufficient, whereby wear resistance of a sintered compact will be insufficient. Accordingly, the amount of the Fe alloy powder B with respect to the Fe alloy powder A is set to be 40 to 60%.

Thus, in the production method for sintered machine components of the present invention, the raw powder is a mixed powder in which 40 to 60% of the Fe alloy powder B is added to the Fe alloy powder A, and 1.0 to 5.0% of the Fe—P alloy powder and 0.5 to 3.5% of the graphite powder are also added thereto. The Fe—P alloy powder consists of 10 to 30% of P and the balance of Fe and inevitable impurities.

The mixed powder having the above composition is compacted into a predetermined shape by a typical powder metallurgical method and is sintered at 1100 to 1300° C. Thus, the overall composition consists of, by mass %, 16.9 to 40.2% of Cr, 6 to 18% of Ni, 0.3 to 1.8% of Mo, 0.3 to 1.8% of Si, 0.1 to 1.5% of P, 1 to 5.2% of C, and the balance of Fe and inevitable impurities. As a result, a sintered machine component having a metallic structure, in which fine granular carbides are dispersed in an austenite matrix, is obtained. A sintered machine component obtained by the production method of the present invention has a density ratio of 95% or more because liquid shrinkage occurs in sintering. Therefore, oxidation of pores and pitting corrosion are reduced, whereby corrosion resistance of the sintered machine component is further improved. Since the matrix structure is an austenite structure, the sintered machine component has superior high-temperature strength and corrosion resistance and has a thermal expansion coefficient equivalent to that of an austenitic heat-resistant steel. Fine granular Cr carbides are dispersed in the matrix, whereby the wear resistance and the corrosion resistance of the sintered machine component are improved. Unlike a material in which Cr carbides are precipitated at grain boundaries, such as a high-chrome cast steel, the fine granular Cr carbides are precipitated within crystal grains, whereby wear resistance and corrosion resistance are sufficiently obtained.

According to the production method for sintered machine components of the present invention, heat resistance, corrosion resistance, wear resistance, and high-temperature strength can be improved, and a sintered machine component having a thermal expansion coefficient equivalent to that of an austenitic heat-resistant steel is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view showing a turbo component of an embodiment of the present invention.

FIG. 2 is a top view showing a turbo component of an embodiment of the present invention.

PREFERRED EMBODIMENT OF THE INVENTION

FIGS. 1 and 2 show an embodiment of the present invention. FIG. 1 is a sectional side view showing a part of a turbocharger for an internal combustion engine, and FIG. 1 shows a reference numeral 2 indicating a nozzle body. A turbine 3 is rotatably supported by a bearing (not shown in the figure) at the center of the nozzle body 2. An end portion of the turbine 3 at a side opposite to a side with nozzle vanes is connected to a compressor (not shown in the figure).

The nozzle body 2 in the above structure is an example of a wear resistant component of the present invention. As shown in FIG. 2, the nozzle body 2 has a ring shape and is formed with plural bearing holes 2 a at the periphery thereof. The bearing hole 2 a rotatably supports a shaft 5 of a nozzle vane 4. A link 6 (only one piece is shown in FIG. 2) is fixed to an end portion of the shaft 5 at the opposite side of the nozzle vane 4. When each link 6 is uniformly driven, the nozzle vanes 4 are turned, whereby the amount of exhaust gas flowing from the outer circumference into the turbine 3 is adjusted. In addition to the above-described nozzle body 2, the wear resistant component of the present invention includes a component that may be mounted to the nozzle body as necessary, such as a nozzle plate, and this component is made of the above-described sintered alloy.

EXAMPLES First Example

Hereinafter, practical examples of the present invention are described in detail. In the following description, each of the “%” symbols represents “mass %”. An Fe alloy powder A (Fe alloy powder disclosed in Japanese Patent No. 3784003), an Fe alloy powder B, an Fe-20P alloy powder, and a graphite powder were prepared. The Fe alloy powder A consisted of 30% of Cr, 2% of Mo, 2% of Si, and 1% of C, the Fe alloy powder B consisted of 25% of Cr, 20% of Ni, and the balance of Fe and inevitable impurities, and the Fe-20P powder included 20% of P. An amount from 30 to 70% of the Fe alloy powder B was added to the Fe alloy powder A, and 2.5% of the Fe-20P powder and 2.7% of the graphite powder were added thereto, whereby a mixed powder was obtained. The mixed powder was compacted at a compacting pressure of 600 MPa into a pillar shape having an outer diameter of 10 mm and a height of 10 mm, whereby a green compact was obtained. The green compact was sintered at 1200° C. for 60 minutes in an ammonia decomposed gas atmosphere, whereby samples Nos. 1 to 5 were formed. On the other hand, the Fe alloy powder B was not added, but 2.5% of the Fe-20P powder and 1.0% of the graphite powder were added to the above Fe alloy powder A, whereby a raw powder was prepared. By using this raw powder as a conventional material (material disclosed in Japanese Patent No. 3784003), a sample No. 6 was prepared in the same manner as the above manner. An ingot material of a high-chrome cast steel was worked into the above shape, whereby a sample No. 7 was prepared. The high-chrome cast steel consisted of 34% of Cr, 2% of Mo, 0.2% of Ni,2% of Si, 1.2% of C, and the balance of Fe and inevitable impurities.

These samples were heated at a temperature in a range of 700 to 900° C. for 100 hours in an air atmosphere, and increases in the weights thereof after heating were measured. The results are shown in Table 1. Each sample was heated at 800° C., and tensile strength (high-temperature strength) and thermal expansion coefficient were measured. These results are also shown in Table 1.

TABLE 1 Oxidized High- Mixing ratio mass % amount temperature Thermal Fe alloy Fe alloy (Air × 100 hr) strength expansion Sample powder A powder B Fe—20P Graphite g/m² (800° C.) coefficient No. (Fe—30Cr—2Mo—2Si—1C) (Fe—25Cr—20Ni) powder powder 700 800 900 MPa 10⁻⁶ K⁻¹ Notes 1 Balance 30.0 2.5 2.7 2 7 13 210 15.8 Exceeds lower limit of amount of alloy powder B 2 Balance 40.0 2.5 2.7 2 7 11 250 16.1 3 Balance 50.0 2.5 2.7 2 7 12 300 16.2 4 Balance 60.0 2.5 2.7 2 7 12 280 16.2 5 Balance 70.0 2.5 2.7 2 7 13 250 16.3 Exceeds upper limit of amount of alloy powder B 6 Balance — 2.5 1.0 1 4 9 220 12.4 Conventional material Japanese Patent No. 3784003 7 Fe—34Cr—2Mo—0.2Ni—2Si—1.2C 5 13 47 180 12.3 Conventional (Ingot material) material High-chrome cast steel (1) Effects of amount of Fe alloy powder B

As shown in Table 1, the samples Nos. 1 to 5 including the Fe alloy powder B exhibited increases in the weights due to oxidation, which were approximately the same as that of the sample No. 6 of a conventional material. Therefore, the samples Nos. 1 to 5 were superior in oxidation resistance. The sample No. 1 including less than 40% of the Fe alloy powder B exhibited smaller high-temperature strength than that of the sample No. 6 of the conventional material. On the other hand, in the samples Nos. 2 to 5 including more than 30% of the Fe alloy powder B, the high-temperature strengths were improved and were higher than that of the sample No. 6 of the conventional material. In the sample No. 5 including more than 60% of the Fe alloy powder B, the high-temperature strength was decreased. In the samples Nos. 2 to 5 including more than 30% of the Fe alloy powder B, the matrixes were completely austenized, and the thermal expansion coefficients were approximately same. Accordingly, by adding 40 to 60% of the Fe alloy powder B to the conventional Fe alloy powder A, high-temperature strength is improved, and the matrix composition is completely austenized without decreasing oxidation resistance.

Second Example

Fe alloy powder B having a composition shown in Table 2 was prepared. Then, 50% of the Fe alloy powder B, 2.5% of the Fe-20P powder, and 2.7% of the graphite powder were added to the Fe alloy powder A in the first example, and they were mixed together into a mixed powder. The mixed powder was compacted and was sintered in the same manner as in the first example, whereby samples Nos. 8 to 18 were formed. In these samples, increases in the weights due to oxidation, tensile strength (high-temperature strength), and thermal expansion coefficient were measured in the same manner as in the first example. These results and the results of the sample No. 3 in the first example are shown in Table 2.

TABLE 2 Oxidized High- amount temperature Thermal Sam- Mixing ratio mass % (Air × 100 hr) strength expansion ple Fe alloy Fe alloy powder B Fe—20P Graphite g/m² (800° C.) coefficient No. powder A Fe Cr Ni powder powder 700 800 900 MPa 10⁻⁶ K⁻¹ Notes 8 Balance 50.0 Balance 10.0 20.0 2.5 2.7 11 26 52 270 16.4 Exceeds lower limit of Cr amount in alloy powder B 9 Balance 50.0 Balance 15.0 20.0 2.5 2.7 5 15 27 290 16.4 10 Balance 50.0 Balance 20.0 20.0 2.5 2.7 3 9 18 290 16.3 3 Balance 50.0 Balance 25.0 20.0 2.5 2.7 2 7 12 300 16.2 11 Balance 50.0 Balance 30.0 20.0 2.5 2.7 2 6 10 300 16.1 12 Balance 50.0 Balance 35.0 20.0 2.5 2.7 2 5 9 310 16.0 13 Balance 50.0 Balance 40.0 20.0 2.5 2.7 2 4 9 280 15.9 Exceeds upper limit of Cr amount in alloy powder B 14 Balance 50.0 Balance 25.0 10.0 2.5 2.7 2 7 12 220 15.5 Exceeds lower limit of Ni amount in alloy powder B 15 Balance 50.0 Balance 25.0 15.0 2.5 2.7 2 7 12 250 16.0 3 Balance 50.0 Balance 25.0 20.0 2.5 2.7 2 7 12 300 16.2 16 Balance 50.0 Balance 25.0 25.0 2.5 2.7 2 7 12 310 16.2 17 Balance 50.0 Balance 25.0 30.0 2.5 2.7 2 7 12 320 16.3 18 Balance 50.0 Balance 25.0 35.0 2.5 2.7 2 7 12 300 16.4 Exceeds upper limit of Ni amount in alloy powder B (1) Effects of Cr amount in Fe alloy powder B

As shown in Table 2, in the sample No. 8 including less than 15% of Cr in the Fe alloy powder B, the increase in the weight due to oxidation was large. As the Cr amount was increased, the increase in the weight was decreased, and the oxidation resistance was improved. When the Cr amount in the Fe alloy powder B was small, the amount of Cr solid solved in a matrix after Cr forms carbides might be insufficient, whereby oxidation resistance was decreased. In the experiments performed at 700° C., when the Cr amount in the Fe alloy powder B was 15% or more, the effect for improving the oxidation resistance was large. In the experiments performed at 800° C. or 900° C., when the Cr amount in the Fe alloy powder B was 20% or more, the effect for improving the oxidation resistance was large. When the Cr amount in the Fe alloy powder B was from 10 to 40%, each sample exhibited superior high-temperature strength. In this case, in the sample No. 13 including 40% of Cr, the hardness of the Fe alloy powder B was increased, whereby the compressibility of the raw powder was decreased, and the high-temperature strength was decreased. The thermal expansion coefficients were approximately same when the Cr amount in the Fe alloy powder B was from 10 to 40%. Accordingly, when the Cr amount in the Fe alloy powder B is 15 to 35%, preferably, 20 to 35%, superior oxidation resistance and high-temperature strength, and a predetermined thermal expansion coefficient are obtained.

(2) Effects of Ni Amount in Fe Alloy Powder B

As shown in Table 2, when the Ni amount in the Fe alloy powder B was 10 to 35%, the increases in the weights due to oxidation were the same, and superior oxidation resistances were obtained. On the other hand, in the sample No. 14 including 10% of Ni in the Fe alloy powder B, the high-temperature strength was not improved (see the result of the sample No. 6 in the first embodiment). In the samples including 15% or more of Ni, the high-temperature strengths were improved. However, in the sample No. 18 including 35% of Ni, the high-temperature strength was decreased. In the sample No. 14 including 10% of Ni in the Fe alloy powder B, the matrix structure was not completely austenized, whereby the thermal expansion coefficient was small. On the other hand, in the samples including 15% or more of Ni, the thermal expansion coefficients were approximately same. Accordingly, when the Ni amount in the Fe alloy powder B is 15 to 30%, superior oxidation resistance and high-temperature strength, and a predetermined thermal expansion coefficient are obtained.

Third Example

Next, 50% of the Fe alloy powder B in the first example, the Fe-20P powder, and the graphite powder were added to the Fe alloy powder A in the first example, and they were mixed together into a mixed powder. The amounts of the Fe-20P powder and the graphite powder were varied as shown in Table 3. The mixed powder was compacted and was sintered in the same manner as in the first example, whereby samples Nos. 19 to 31 were formed. In these samples, increases in the weights due to oxidation, tensile strength (high-temperature strength), and thermal expansion coefficient were measured in the same manner as in the first example. These results and the results of the sample No. 3 in the first example are shown in Table 3.

TABLE 3 Oxidized High- Mixing ratio mass % amount temperature Thermal Fe alloy Fe alloy (Air × 100 hr) strength expansion Sample powder A powder B Fe—20P Graphite g/m² (800° C.) coefficient No. (Fe—30Cr—2Mo—2Si—1C) (Fe—25Cr—20Ni) powder powder 700 800 900 MPa 10⁻⁶ K⁻¹ Notes 19 Balance 50.0 2.5 0.3 1 3 5 180 15.5 Exceeds lower limit of amount of graphite powder 20 Balance 50.0 2.5 0.5 1 3 5 230 15.9 21 Balance 50.0 2.5 0.8 1 3 6 240 16.0 22 Balance 50.0 2.5 1.0 1 4 7 250 16.0 23 Balance 50.0 2.5 1.5 2 4 8 270 16.0 24 Balance 50.0 2.5 2.0 2 5 9 280 16.1 25 Balance 50.0 2.5 2.5 2 6 11 300 16.1 3 Balance 50.0 2.5 2.7 2 7 12 300 16.2 26 Balance 50.0 2.5 3.5 5 10 22 300 16.3 27 Balance 50.0 2.5 4.0 — — — — — Exceeds upper limit of amount of graphite powder, test piece could not be formed 28 Balance 50.0 0.0 2.7 24 42 79 120 16.2 Exceeds lower limit of amount of Fe—P powder 29 Balance 50.0 0.5 2.7 4 9 16 240 16.2 3 Balance 50.0 2.5 2.7 2 7 12 300 16.2 30 Balance 50.0 5.0 2.7 4 8 15 280 16.2 31 Balance 50.0 10.0 2.7 — — — — — Exceeds upper limit of amount of Fe—P powder, test piece could not be formed (1) Effects of amount of graphite powder

As shown in Table 3, as the amount of the graphite powder was increased, the amount of chrome carbides precipitated in the matrix was increased, and the Cr amount in the matrix was decreased, whereby the oxidation resistance was decreased. The increases in the weights due to oxidation were within a practical range. In the sample No. 27 including 4.0% of the graphite powder, the compressibility of the powder was decreased, and the sample could not be formed. The sample No. 19 including 0.3% of the graphite powder exhibited small high-temperature strength. As the amount of the graphite powder was increased, the high-temperature strength was improved. The sample No. 19 including 0.3% of the graphite powder exhibited small thermal expansion coefficient. On the other hand, in the samples including 0.5% or more of the graphite powder, the matrix structures were substantially completely austenized, whereby the thermal expansion coefficients were increased. Accordingly, the lower limit of the amount of the graphite powder is preferably set to be 0.5% (more preferably, 0.8%), and the upper limit of the amount of the graphite powder is preferably set to be 3.5%.

(2) Effects of Amount of Fe—P Alloy Powder

As shown in Table 3, in the sample No. 28 that did not include the Fe—P alloy powder, a liquid phase was not generated, and density of a sintered compact was not increased, whereby the oxidation resistance and the high-temperature strength were extremely decreased. On the other hand, in the samples including 0.5% or more of the Fe—P alloy powder, Fe—P liquid phases were generated, and the densities of the sintered compacts were improved, whereby the oxidation resistances and the high-temperature strengths were improved. In the sample No. 31 including 10% of the Fe—P alloy powder, too much of the liquid phase was generated, whereby the sintered compact was deformed, and the sample could not be formed. Accordingly, when the amount of the Fe—P alloy powder is in a range of 0.5 to 5%, a satisfactory sintered compact can be produced. 

1. A production method for sintered machine components, comprising: preparing an Fe alloy powder A, an Fe alloy powder B, an Fe—P powder, and a graphite powder, the Fe alloy powder A consisting of, by mass %, 25 to 45% of Cr, 1.0 to 3.0% of Mo, 1.0 to 3.0% of Si, 0.5 to 1.5% of C, and the balance of Fe and inevitable impurities, the Fe alloy powder B consisting of, by mass %, 15 to 35% of Cr, 15 to 30% of Ni, and the balance of Fe and inevitable impurities, and the Fe—P powder consisting of 10 to 30 mass % of P and the balance of Fe and inevitable impurities; mixing 40 to 60 mass % of the Fe alloy powder B, 1.0 to 5.0 mass % of the Fe—P powder, and 0.5 to 3.5 mass % of the graphite powder with the Fe alloy powder A into a mixed powder; compacting the mixed powder into a green compact; and sintering the green compact. 